Answer:
Study, study, study! I can’t really tell you what not to study, but focus most on subjects you aren’t the best at. Say your best subject is history. You don’t want to study hard on history when you already know all that. Study your weak points. Good luck, and I hope this answers your question!
Answer:
a. on
b. I don't think here a preposition is needed as "This is the key to my room" is a complete sentence.
c. in
d with
Hope it helps!!
Have a nice day:)
